1. Field of the Invention
This invention relates to imaging a specimen using charged particle beam imaging devices. In particular, the invention relates to the design of charged particle detectors for article imaging devices that utilize charged particle beams. More specifically, the invention relates to the design of low-profile electron detectors for charged particle beam imaging devices.
2. Description of the Related Art
In a conventional charged particle beam apparatus, a specimen under inspection is irradiated using a primary electron beam. The interaction of the primary electron beam with the specimen causes the latter to emit electrons with kinetic energies ranging between zero electron-volts (eV) and the kinetic energy of the electrons in the primary beam.
The electrons emitted by the specimen are classified according to their initial kinetic energies. The first group of electrons with kinetic energies of up to 50 eV are called secondary electrons, or secondaries. The secondary electrons emitted by the specimen typically carry the information about the topographical structure of the specimen.
The interaction of the primary electron beam with the specimen also causes the emission of the second class of electrons, called backscattered electrons. The backscattered electrons have energies ranging from 50 eV and up to the kinetic energy of the electrons in the primary electron beam and carry information about the material composition of the specimen.
The secondary and backscattered electrons emitted by the specimen are collected using an electron detector. It should be noted that most of the existing electron detectors are capable of detecting only electrons with kinetic energies included in a predetermined detection energy range. In addition, the detection efficiency (the ratio of the number of detected electrons to the total number of secondary electrons emitted from the specimen) may depend on the electron energy. Accordingly, in order to detect the secondary and backscattered electrons with higher efficiency, their kinetic energies can be further increased. Typically, this is accomplished by accelerating the electrons in the electric field of the particle beam apparatus. The aforementioned accelerating electric field can be produced by biasing the surface of the specimen and the surface of the electron detector, such as to create a suitable electric potential difference therebetween.
The electron detector collects the electrons emitted by the specimen and generates an output electrical signal representative of the cumulative charge of the collected electrons, multiplied by the amplification factor of the detector. The electric signal produced by the electron detector is used in creating an image of the specimen. Depending on the nature of the electrons used in imaging (secondary or backscattered), the created image is indicative of either the topographic or material structure of the specimen. After the image of the area of the specimen irradiated by the primary beam spot is created using the secondary and/or backscattered electrons, the specimen is moved with respect to the irradiating primary electron beam so that the particle beam apparatus can produce an image of the next area. The specimen can be moved in a continuous or stepwise manner.
Unfortunately, the secondary and backscattered electrons emitted by the specimen typically have a wide kinetic energy distribution. Therefore, the amount of time required for various electrons to reach the electron detector of the column varies substantially. A wide dispersion in the arrival times of the secondary electrons emitted from the specimen results in a decrease of the scanning speed of the particle beam apparatus because the apparatus has to xe2x80x9cwaitxe2x80x9d for the slowest electrons emitted by the irradiated spot of the specimen to reach the detector, before it can move on to scan the next spot. Because the width of the collection time distribution is proportional to the average electron travel time, it is highly advantageous to minimize the electron travel time by accelerating electrons in an electric field.
Various electron detection schemes for electron beam imaging systems have been developed. One of these recently developed methods is an indirect electron detection scheme. For example, one type of Applied Materials scanning electron microscope (SEM) column uses indirect detection whereby secondary electrons emitted from the specimen first strike a conical structure near the face of a micro channel plate (MCP) detector. Secondary electrons generated on the cone are attracted to the face of the MCP via an electric field. This design allows for the collection of secondary electrons that are close to the optic axis, thus presenting a challenge in using an MCP detector since a small aperture (typical MCPs are limited to not less than 0.3 mm) is difficult to manufacture in this type of detector.
An indirect electron detection concept also is known in which electrons emitted from the specimen first strike a target electrode having a small aperture and newly-generated electrons (tertiary electrons) are accelerated toward a rear facing detector, such as a micro channel plate detector (MCP). The use of the target permits directing the tertiary electrons away from the optical axis of the apparatus. Therefore, the system is capable of efficient detection of the electrons traveling close to the optical axis. In addition, the tertiary electrons land over a relatively wide area on the surface of the electron detector, which improves the detector""s aging characteristics. However, it is desirable in the charged particle beam apparatus to further accelerate the electrons before detecting and to correct the aberration of the secondary electron beam.
Pursuant to the foregoing discussion, there is a strong and widely recognized need for, and it would be highly advantageous to have an improved charged particle beam apparatus that would provide for higher electron landing energies, would correct aberration of the secondary electron beam, and would provide for efficient detection of secondary electrons traveling close to the optical axis.
It is therefore one feature of the invention to overcome the above shortcomings of known techniques by providing an improved charged particle beam apparatus yielding higher electron landing energies, correcting aberration of the secondary electron beam, and efficiently detecting secondary electrons traveling close to the optical axis.
To achieve the above and other features and realize the benefits and advantages of the invention, there is provided a method and system for imaging a specimen in a charged particle beam apparatus using an electron mirror.
According to the inventive method, the specimen is induced to emit electrons. This can be done, for example, by irradiating the specimen with a primary electron beam. The inventive method further comprises deflecting the emitted electrons in a field of an electron mirror and detecting the emitted electrons.
Another aspect of the present invention is a charged particle beam apparatus using an electrostatic mirror. The inventive apparatus comprises an electron beam generation and transport system for providing a primary electron beam. The primary electron beam irradiates the specimen and induces the specimen to emit electrons. The apparatus also includes an electron mirror for deflecting emitted electrons and an electron detector for detecting the deflected emitted electrons to create an image of the specimen.
Further improvements include providing a primary electron beam shield for shielding the primary electron beam and the emitted electrons from the electric field of the electron mirror.
Still further improvements include providing a second electron mirror for controlling the landing direction of the emitted electrons on the surface of the electron detector.